The optical properties of electrochromic materials change in response to electrically driven changes in oxidation state. Thus, when an applied voltage from an external power supply causes electrons to flow to (reduction) or from (oxidation) an electrochromic material, its transmittance properties change. In order to maintain charge neutrality, a charge balancing flow of ions in the electrochromic device is needed. By enabling the required electron and ion flows to occur, an electrochromic device utilizes reversible oxidation and reduction reactions to achieve optical switching.
Conventional electrochromic devices comprise at least one thin film of a persistent electrochromic material, i.e. a material which, in response to the application of an electric field of given polarity, changes from a high-transmittance, non-absorbing state to a low-transmittance, absorbing or reflecting state. Since the degree of optical modulation is directly proportional to the current flow induced by the applied voltage, electrochromic devices demonstrate light transmission tunability between high-transmittance and low-transmittance states. In addition, these devices exhibit long-term retention of a chosen optical state, requiring no power consumption to maintain that optical state. Optical switching occurs when an electric field of reversed polarity is applied.
To facilitate the aforementioned ion and electron flows, an electrochromic film which is both an ionic and electronic conductor is in ion-conductive contact, preferably direct physical contact, with an ion-conducting material layer. The ion-conducting material may be inorganic or organic, solid, liquid or gel, and is preferably an organic polymer which also serves as a laminating agent. The electrochromic film(s) and ion-conductive material are disposed between two electrodes, forming a laminated cell.
When the electrode adjacent to the electrochromic film is the cathode, application of an electric field causes darkening of a cathodically-coloring film. Reversing the polarity causes electrochromic switching, and the film reverts to its high-transmittance state. Typically, an electrochromic film such as tungsten oxide is deposited on a substrate coated with an electroconductive film such as tin oxide or indium tin oxide to form one electrode. The counter electrode is typically a similar tin oxide or indium tin oxide coated substrate.
A typical laminated electrochromic device, such as an electrochromic lens, comprises a first electroconductive material layer (which serves as a first electrode) an electrochromic layer, an ion-conducting material layer and a second electroconductive layer (which serves as a second electrode). Preferably, a complementary electrochromic layer is also used. These electroconductive and electrochromic layers, along with the ion-conducting material layer, can be arranged as a single stack deposited on a first substrate which is then laminated to a second substrate, or they can be arranged such that the electrodes are coated on separate substrates, followed by placement of one or more electrochromic layers on the substrates. The coated substrates are then laminated via a technique which positions an ion-conducting material between them. Preferably, an ion-conducting polymer, which also serves as a bonding agent, is used to bond the complementary substrates.
As voltage is applied across the electrodes, ions are conducted through the ion-conducting material. To ensure reliable operation, the peripheral edge surface of the ion-conducting material layer generally must be sealed so as to maintain its water content within a range sufficient to provide the required ion conductivity. Absent an adequate seal, moisture loss or gain through the exposed edge of the ion-conducting material layer impacts performance.
The peripheral edge surface of a laminated electrooptic device may be shaped to support or interlock with an edge seal. For example, copending and commonly owned U.S. patent application Ser. No. 08/995,788, filed Dec. 22, 1997, now U.S. Pat. No. 5,969,847, discloses a nubbed-edge design that facilitates application of an edge seal to the peripheral edge surface of an electrochromic device. Also, copending and commonly owned U.S. patent application Ser. No. 08/996,064 (now U.S. Pat. No. 5,953,150) and Ser. No. 09/046,386, filed on Dec. 22, 1997 and Mar. 23, 1998, respectively, disclose methods for sealing laminated electrochromic devices.
U.S. Pat. No. 5,327,281 to Cogan discloses the use of epoxy to seal a cavity formed when a spacer is used to separate electrodes and contain a liquid electrolyte injected between the spaced electrodes.
U.S. Pat. No. 5,657,150 to Kallman et al., discloses an electrochromic device having an isolative barrier which electrically isolates the device electrodes.